This disclosure relates to the field of electric submersible pumps (ESPs) used in subsurface wells. More specifically, the disclosure relates to sensors used in connection with ESPs to monitor motor speed and other functional parameters of the ESP and the ambient environment in the well.
Submersible pumping systems such as ESPs are deployed in wells to recover petroleum fluids from subterranean reservoirs and to remove water from gas wells to enhance productivity, among other uses for such pumping systems. A submersible pumping system such as an ESP may include a number of components, such as a fluid filled electric motor coupled to one or more high performance pumps. “Monitoring packages” (i.e., various forms of sensor assemblies in suitable housings for use in a subsurface well) are used to provide the well operator and an automated control system with real-time information about the performance of the submersible pumping system and the ambient environment in the well.
Different arrangements of a submersible pumping system are possible, including disposing the (electric) motor below the pump, in which case the monitoring package may conveniently be placed below the motor. In other pumping systems, the motor may be disposed above the pump, in which case the monitoring package will be either above the motor (and whereby the motor power cable has to pass the monitoring package), or the monitoring package may be inline, disposed longitudinally between the motor and the pump, in which case the motor shaft has to pass through the monitoring package in order to drive the pump.
Submersible electric motors are known to use three-phase AC power to drive the motor, using one of several wiring configurations known in the art. Such wiring configurations include a wye connection in which three conductors share a common neutral connection. The wye connection may conveniently provide a source of power for the monitoring package using the power lines connected to the electric motor. In this way, a single connection from a surface-located motor control unit can be used to control the speed of and provide electric power to the motor, and to provide electric power to the monitoring package. Power for the monitoring package may be provided as simply as using the common connection of the wye point, such that electrical power may be provided to the monitoring package.
Sensor measurement data from the monitoring package may conveniently be transmitted to surface along the motor power cable, most conveniently by using the common or wye connection from one or more phases of the motor power supply lines.
U.S. Pat. No. 8,347,953 issued to Elizondo et al. explains an inline monitoring package in which the interior of the monitoring package is filled with lubricating oil, and the electronic components of the monitoring package are encapsulated to prevent contact with lubricants within the inline monitoring package. The method of encapsulation is to submerge electronic components into a resin system, for example, an epoxy resin, and allowing the resin to cure. After curing, a thick coating of the resin completely surrounds the components to form an impervious seal from the surrounding conditions. This process is also called “potting.”
Monitoring packages are known to include an inductor which may be used to monitor the rotational speed of the motor shaft and can also be configured to detect lateral movement or vibration in the shaft as it rotates. The '953 patent further discloses a mechanical seal and a fluid exchange system that includes a series of passages that permit the movement of fluid between the motor and the pump.
The potting method of encapsulation of the electronic components can cause difficulties because the components may be subjected to high ambient pressure at a substantial vertical depth in the well. Such pressure is exerted on the exterior of the resin encapsulation and will be communicated to the electronic component through the encapsulation material. Such high pressure can cause damage or malfunction of the electronic components. Also, the reliability of the electronic components depends on the integrity of the encapsulation. If the encapsulation is not completely impermeable and without cracks or other imperfections, or if the encapsulation deteriorates (e.g., due to adverse conditions at depth in the wellbore, such as high temperature, vibration, high pressure, etc.) fluid may come into contact with one or more of the electronic components and disrupt correct functioning of the monitoring system.
Further, many electronic components which are preferable for use in the monitoring system may be qualified only for use in air.
Further, encapsulated electronic components cannot be serviced, for example, by replacing some components, or by repairing connections between components or between components and wiring. As a result, encapsulated electronic components in a monitoring system may require that certain expensive electronic components are scrapped if the monitoring system fails, when economic repair of the monitoring system may otherwise have been possible.
An inductor disclosed in the '953 patent to monitor shaft rotation speed and vibration is a large and costly component. For high performance, small diameter ESP systems, a preferred type of electric motor is a permanent magnet motor. Such motors are synchronous, that is, the motor speed is controlled by the frequency of the electrical power supply. Therefore, the inductor of such systems as shown in the '953 patent is not required to monitor rotational speed of the shaft when a permanent magnet motor is used in a pump system.
An example of a sensor system such as the one disclosed in the '953 patent is shown schematically in FIG. 2.